The present invention relates to a liquid crystal display device and its manufacturing method, and more particularly to an active matrix type liquid crystal display device having active elements such as thin film transistors or the like and its manufacturing method.
The liquid crystal display device substantially uses a liquid crystal panel which sandwiches a liquid crystal layer between two sheets of substrates, wherein at least one substrate is made of transparent glass or the like. The liquid crystal display device is roughly classified into a type which performs lighting and lights-out of given pixels by selectively applying voltage to electrodes for forming pixels provided to these two substrates respectively (a so-called simple matrix type) and a type which performs lighting and lights-out of given pixels by forming a liquid crystal panel which disposes active elements (switching elements) for selecting pixels on the above-mentioned seed electrodes and then selecting these active elements (for example, an active matrix type which uses thin film transistors (TFT) as active elements).
Particularly, the latter active matrix type liquid crystal display devices have become the main stream of the liquid crystal display devices because of their high contrast performance, their high speed display performance and the like.
This active matrix type liquid crystal display device consists of a vertical electric field system which applies an electric field for changing the orientation direction of the liquid crystal layer between electrodes formed on one substrate and electrodes formed on the other substrate and a transverse electric field system (IPS: In-Plane Switching Mode) which applies an electric field for changing the orientation direction of the liquid crystal layer between electrodes formed on only one substrate in a direction parallel to the substrate.
In these liquid crystal display devices, signal wirings for supplying scanning signals and video signals to respective electrodes of the liquid crystal panel are formed such that the signal wirings extend from a display region of the liquid crystal panel to a terminal portion of an end periphery of the same panel.
Drive circuit chips (drive ICs) at terminal portions of respective wirings or a connection portion to which a flexible printed circuit board mounting the drive Ics is connected are coated with a conductive film made of an oxide so as to prevent inconveniences such as the increase of the contact resistance.
The liquid crystal display device of the vertical electric field type adopts an oxide transparent conductive film as such an oxide conductive film and the terminal portions of the wirings are coated with this oxide transparent conductive film.
In the liquid crystal display device of the transverse electric field type, however, since the orientation direction of the liquid crystal layer is changed by forming an electric field between electrodes (between a pixel electrode and a counter electrode) formed in parallel with the substrate, transparent electrodes are unnecessary.
On the other hand, to ensure the reliability of the terminal portions, that is, to prevent an electrolytic corrosion which oxidizes and gives rise to corrosion and dissolution of wiring and electrode forming materials upon supply of electricity, the terminal portions are coated with a transparent conductive film made of a chemically stable oxide.
As documents which disclose the liquid crystal display device of this transverse electric field system, Japanese patent publication Sho 63-21907, U.S. Pat. No. 4,345,249 and the like can be listed.
As described heretofore, in the liquid crystal display device of the transverse electric field system, although it is unnecessary to form the transparent conductive film on the pixel region, the transparent conductive film made of an oxide becomes necessary at the terminal portions to ensure the connection stability. Accordingly, the display region, all signal wirings at terminal portions and respective electrodes of the liquid crystal display device must be constructed such that a transparent conductive film is coated on an upper layer of a metal film and this gives rise to a drawback in reducing the number of steps of an etching (wet etching) by photolithography.
Besides the patterning of the metal film at the terminal portions, additional one photolithography step and one etching step become necessary for the patterning of transparent conductive film made of an oxide film so that the patterning of the terminal portions requires the photolithography step at least twice.
It is an object of the present invention to overcome the above-mentioned drawbacks of the prior art and to provide a liquid crystal display device and its manufacturing method which can simplify the patterning step of the signal wirings and the electrodes of the liquid crystal display device of the transverse electric field system and can enhance the reliability by preventing the electrolytic corrosion or the like of the terminal portions.
To achieve the above object, the present invention is characterized by realizing a liquid crystal display device of the transverse electric field system which is provided with signal wirings and various kinds of electrodes which are formed by patterning by making use of the difference of corrosion speed (etching rate) due to the corrosion potential difference between different kinds of metals.
The typical constitution of the liquid crystal display device of the present invention is described as follows.
That is,
(1) Electrodes and/or signal wirings of a laminated structure which arranges an amorphous oxide conductive film having a large etching rate on a metal film are formed.
(2) In a liquid crystal display device wherein the device is comprised of one substrate on which gate electrodes which apply scanning signals through scanning signal wirings, drain electrodes which apply video signals through video signal wirings, thin film transistors which has source electrodes and counter electrodes are formed and the other substrate which has at least color filters, and a liquid crystal layer is sealed between one substrate and the other substrate,
the whole or a portion of the scanning signal wirings, the video signal wirings, the gate electrodes, the drain electrodes, the source electrodes and counter electrodes are formed into a laminated structure which arranges an amorphous oxide conductive film having a large etching rate on an upper layer of a metal film.
(3) The metal film of (1) or (2) is formed by laminating two kinds of materials having different corrosion potentials.
(4) On the signal wirings and/or various kinds of electrodes, a thin film having a laminated structure which arranges an amorphous oxide conductive film having a lower corrosion potential than a metal film in an etchant on the upper surface of a metal film is formed and this thin film having a laminated structure is subjected to a desired patterning in an etching treatment step which performs photolithography only once.
(5) The metal film having a laminated structure of (4) is chromium or a chromium-molybdenum alloy and the oxide conductive film is an indium-tin-oxide.
(6) As the material of the amorphous oxide conductive film, an oxide of indium (In) or zinc (Zn) is used.
As material of the metal film, chromium (Cr), chromium-molybdenum (Crxe2x80x94Mo), chromium-tungsten (Crxe2x80x94W) or the like can be used.
The reason that the object of the present invention is achieved by the above-mentioned constitution is as follows. That is, in manufacturing an active matrix type liquid crystal panel which constitutes a liquid crystal display device of a transverse electric field system, on one substrate out of two substrates, gate wirings (scanning signal wirings), drain wirings (video signal wirings), source electrodes (although the source electrodes are connected to pixel electrodes, the source electrodes includes such pixel electrodes here) and counter electrodes (or called xe2x80x9ccommon electrodesxe2x80x9d) are formed.
These wirings and electrodes are formed in a multi-layered structure where the metal oxide film is laminated to the upper layer of the metal film having a single or two layers. The metal oxide film which constitutes this multi-layered structure ensures the reliability of connection between wiring thereof and wirings of a printed circuit board or drive ICs against the oxidation of the terminals at the terminal portions of wirings thereof. Accordingly, it is no more necessary to add the photolithography exclusively for the protection of the terminal portions.
Further, using the oxide film in the amorphous state as this oxide conductive film, the oxide conductive film can be processed by the same etching together with the metal film which constitutes the lower layer. Etching of the oxide in the crystalline condition by an etchant (etching liquid) having strong acidity for metal wirings is extremely difficult and hence, usually, an etching treatment which uses a different etchant becomes necessary for etching the oxide in the crystalline condition.
This is because that the oxide film in the crystalline state has a high corrosion potential in the etchant so that it is hardly etched in the etchant for metal wirings. To the contrary, the oxide film in the amorphous state has a low corrosion potential even in the etchant for metal wirings and the corrosion potential is substantially equal to that of the metal film.
Accordingly, by optimizing the respective compositions of the etchant, the metal film and the oxide film, it becomes possible to simultaneously etch the metal film and the oxide film by using the same etchant.
With respect to the optimum materials for the combination of above-mentioned respective compositions, as the etchant, for example, an etchant which contains nitric acid second cerium ammonium aqueous solution as a main component or an etchant which contains phosphoric acid, nitric acid or acetic acid as a main component can be used.
As the metal film, pure chromium (Cr) or its alloy, molybdenum (Mo) and its alloy or a laminated film made of these metals can be used.
As the oxide film, that is, the oxide conductive film, an oxide made of indium (In), zinc (Zn) and oxygen (O) can be used. In particular, the Inxe2x80x94Znxe2x80x94O film has an advantage that it can maintain the amorphous structure even at the substrate temperature which is substantially as high as a temperature at the time of forming a chromium (Cr) film or a molybdenum (Mo) film. Furthermore, since the film forming temperature is high, the adherence of the oxide conductive film with the metal film can be ensured.
At the time of performing the wet etching of laminated wirings made of the oxide conductive film and the metal film, in addition to an advantage that they can be etched by using the same etchant, the end surface shape of the wirings can be formed into a normally tapered shape by adjusting respective etching rates.
By forming the end surface of wirings into a normally tapered shape, the coverage of the film which is coated on the upper portion thereof can be improved. That is, by applying a tapering processing to gate wirings and the counter wirings which constitute the lowermost portion, the shape of gate insulation films, a-Si films (semiconductor layers), the drain wirings and the source wirings (electrodes) can be similarly formed into a normally tapered shape and the coverage of a passivation film (a PAS film) which constitutes the uppermost layer can be improved so that the corrosion of wirings caused by the poor coverage of the PAS film can be prevented.
Further, since the uppermost portions of all wirings are coated with the oxide, there arises an advantage that the adherence between a photoresist and a wiring film in the photolithography step can be largely enhanced compared to the case that only the metal film is used.
Further, it is needless to say that the present invention is not limited to the above-mentioned constitutions and explanation and various modification are possible without departing from the technical idea of the present invention.